In a typical induction heated cooking apparatus, a high-frequency current is supplied from a frequency inverter to an induction heating coil, which generates a high-frequency magnetic field for subjecting a cooking vessel, such as a metal pan, along with food contained therein, to heat. The induction heating coil constitutes a resonant circuit together with a capacitor. The cooking vessel is placed on the apparatus adjacent to the induction heating coil. The high-frequency magnetic field induces an eddy-current in the body of the cooking vessel. Heat arises in the body of the cooking vessel as an eddy-current loss, due to the skin resistance of the body material of the cooking vessel against the eddy-current. As a result, the food contained in the cooking vessel is cooked by the heat.
When the cooking vessel is removed from the apparatus during or after the cooking operation, the apparatus goes to a non-load state. In the non-load state, the input impedance of the resonant circuit is enormously decreased, so that the high-frequency current in the resonant circuit greatly increases. This phenomenon has been heretofore utilized to detect the presence of the cooking vessel on the apparatus.
The high-frequency current is detected by a current transformer. When the detected current exceeds a predetermined value, a prescribed control circuit deactivates the frequency inverter. As a result, the induction heated cooking apparatus is protected from an erroneous heating operation in the non-load state.
As will be understood from the above description, the heat arises in the body of the cooking vessel due to the skin resistance. Therefore, it is desirable to use a cooking vessel made of high resistance metal. For this reason, the preferred cooking vessel for induction heated cooking is generally made of high resistance metal, such as iron or stainless steel.
In recent years, however, attempts have been made to develop an induction heated cooking apparatus which can operate effectively with cooking vessels made of low resistance copper or aluminum, as well as a vessel made of high resistance iron or stainless steel. To heat such a copper or aluminum vessel effectively, it is necessary to intensify the skin resistance of the body of the cooking vessel and/or the eddy-current induced in the body of the cooking vessel.
The eddy-current can be intensified by increasing the high-frequency current in the resonant circuit. For example, an increase of the input impedance of the resonant circuit is effective to increase the high-frequency current. The skin resistance can be intensified by raising the frequency of the magnetic field caused by the high-frequency current in the resonant circuit. This is because the skin resistance Rs is generally defined as follows: ##EQU1## where F represents the frequency of the high-frequency current, .mu. represents the permeability, and represents the specific resistance of the metal.
Further, the input impedance Z of the resonant circuit is defined as follows: ##EQU2## where K represents a constant, and N represents the number of winding turns of the induction heating coil. The permeability .mu. and the specific resistance r are constant for each metal.
As is seen from the above equation (1), the skin resistance Rs is increased by raising the frequency F. As seen from equation (2), the input impedance Z is increased by increasing the number of turns N and/or raising the frequency F.
Attempts to make an induction heated cooking apparatus which can operate effectively with cooking vessels made of copper or aluminum and iron or stainless steel have resulted in a problem. The presence of a lo resistance cooking vessel on the apparatus could not be accurately detected. This is because the resonant circuit has a similar low input impedance both in the non-load state of the cooking vessel and in the load state of a cooking vessel made of copper or aluminum.
Accordingly, it has been further attempted to discriminate between body materials of cooking vessels made of copper or aluminum and cooking vessels made of iron or stainless steel b using the difference between the resonant frequencies of the resonant circuit for each material. That is, copper or aluminum has a good response for a relatively high resonant frequency, as described above. By contrast, iron or stainless steel has a good response for a relatively low resonant frequency. Thus, it was assumed that a cooking vessel made of copper or aluminum could be discriminated from a cooking vessel made of iron or stainless steel by using the difference between the resonant frequencies.
However stainless steel has two different resonant frequencies. That is, magnetic stainless steel has a low resistance and a low resonant frequency, like iron. However, non-magnetic stainless steel has a low resistance and a high resonant frequency, like copper and aluminum.
Therefore, it has been eagerly desired to develop an induction heated cooking apparatus which can be used with cooking vessels made of many metals, i.e.. iron, magnetic stainless steel, non-magnetic stainless steel, copper or aluminum.